In the preparation of polymers, moist polymer powders which are treated in subsequent drying stages are frequently obtained. The drying of polymer powders has been practiced for a long time. A wide variety of apparatuses, for example drum dryers, screw dryers or fluidized-bed dryers and also an apparatus for spray drying, are used for this purpose.
Thus, for example, DE 602 12 652 T2 describes a method of dewatering and degassing polymer powders by use of an extruder.
DE 10 2007 020 951 A1 discloses a method and an apparatus for purifying a resorbable polyester. Here, a moist polymer composition is dried using a fluidized-bed dryer.
DE 696 33 192 T2 describes a method and an apparatus for drying pulverized material, which is carried out on spiral tracks in a cylindrical tube.
DE 42 40 716 A1 discloses a method of drying vinyl chloride polymers by convective cocurrent drying. The method uses a flow tube and a directly adjoining cyclone dryer unit without additional contact heating.
DE 30 34 983 A1 discloses a method of thermally after-treating polymers of vinyl chloride which can be converted into a paste. The polymer emulsions obtained are subjected to spray drying.
Fluidized-bed granulating dryers in which part of the heat generated in the process is recovered in order to make the overall process more efficient have also been described. Such an apparatus is known from DE 34 45 437 A1.
Many chemical reactions, including polymerization reactions or processes for preparing monomers required therefor, are exothermic. Utilization of the heat of reaction evolved in the chemical reaction or else the waste heat from work-up processes in the preparation of such compounds would improve the energy efficiency of the production processes. However, there is often the problem that the waste heat is obtained at a temperature level which is too low for the use.
1,2-Dichloroethane (hereinafter “DCE”) is used predominantly as intermediate in the preparation of monomeric vinyl chloride (hereinafter “VC”) which is in turn used as intermediate for the preparation of polyvinyl chloride (hereinafter “PVC”). Hydrogen chloride HCl is formed in the conversion of DCE into monomeric vinyl chloride. This hydrogen chloride is preferably used in the preparation of DCE by oxychlorination of ethene by means of HCl and oxygen. An alternative route for the preparation of DCE proceeds via the direct chlorination of ethene by means of chlorine. Both routes are employed in the industrial preparation of DCE, so that a good balance between the following reaction equations is achieved in respect of the hydrogen chloride produced and consumed:Cl2+C2H4→C2H4Cl2+218 kJ/MolC2H4Cl2→C2H3Cl+HCl−71 kJ/MolC2H4+2 HCl+½O2→C2H4Cl2+H2O+238 kJ/Mol
The total balance of the DCE reaction is exothermic. DCE plants are frequently coupled with plants for the preparation of VC and for PVC production.
The polymer obtained in PVC production is frequently moist, especially when the polymerization is carried out as emulsion or suspension polymerization. The polymer powder therefore has to be dried after production and removal of excess liquid. Fluidized-bed dryers are often used for this purpose.